This invention relates to a thermal transfer printer for printing on the surface of a compact disk, particularly a recordable compact disk known as a CD-R disk.
The invention optimizes printing on irregularly shaped media and incorporates features to prevent damage to the thermal transfer print head.
Compact disks are an inexpensive medium for storing digital information that may relate to audio, video and/or any type of information or data that is conveniently stored in digital form. When compact-disks are manufactured in large quantities, the side opposite the recording side of the disk is customarily printed in a mass printing process such as silk screening. The label information applied to the disks is generally identical for each disk and related to the pre-recorded content of the disks.
With the development of the CD-R disk, disks can be sold in blank with the informational content later recorded by a CD-R recorder. In order to appropriately label such disks with regard to the content that is recorded on the disk, programmable disk printers, such as ink jet printers and thermal transfer printers have been devised. These printers print the surface of the disk with graphics and other information that can be customized to correspond to the information recorded on the disk by the CD-R recorder. One drawback in using an inkjet printer is the extended time required to print an individual disk. Another drawback is the additional expense of disk blanks which require a precoated surface for inkjet printing.
Thermal transfer printers can print with greater speed and print on disk blanks prepared with an inexpensive lacquer coating. Thermal transfer printers include a print head that applies a contact pressure to the media to be printed.
One type of thermal transfer printer will typically consist of a mechanism that has a stationary print head, a ribbon, and assembly that moves the media under the print head. The print head contains an array of heating elements. The ribbon is a plastic film with a wax or resin compound deposited on one side. The print head is in contact with the ribbon during printing, and the ribbon is in contact with the media.
By heating the areas of the ribbon, the wax or resin compound is deposited on the media. Printing occurs by moving ribbon and the media at the same rate across the print head, while firing the heating elements in a desired pattern. The print head must exert some pressure on the media for successful transfer of the wax or resin to the media.
A second type of thermal printer is a direct transfer printer, which uses thermally sensitive media that changes color when heated, therefore a ribbon is not required. With thermally sensitive media, the print head marks the media by generating a pattern of heated and non-heated areas on the surface of the media, as it moves under the print head. The invention described is applicable to-both types of thermal printers.
Thermal transfer printers require the print head to contact the printable surface at a uniform pressure for optimum transfer of wax or resin from a ribbon to the media (or heat in the case of direct thermal transfer printer). Variations in print head pressure to the media result in improper printing on media such as non-printed areas or uneven print density.
Printing on rectangular objects, such as a piece of paper is relatively straight forward, since the print head pressure remains constant during the entire printing process. The pressure remains constant because the area of contact between the print head and the media does not change. For example, in printing a 5" wide piece of paper the print head is always in contact with 5" of media. In contrast, printing on an 5" diameter disk, the area of contact would initially be very small as the print head is at the edge of the disk, but then increases to 5" as the print head crosses the center of the disk. After crossing the center of the disk, the area of contact decreases as the print head travels the far edge of the disk.
When the force of the print head applied to the media is constant and the print head travels across a rectangular shaped media, the pressure per unit area is constant. If the print head travels across a disc shaped media, the print head pressure to the media will change as the print head travels across the disc. When the force of the print head applied to the media is constant and the print head travels across a disk shaped media the pressure per unit area changes as the contact area increases and decreases.
To successfully, print on disc shape media, the printer must be constructed to either:
a) vary the force of the print head applied to the media as it travels across the disc to compensate for the variation in width of printable surface, or PA1 b) hold the disc in a manner that effectively presents an unchanging width of contact area for the print head as it moves across the disc.
The process described in point a) can be achieved by using a complicated system of cams, gears, and sensors.
The process described in point b) can be achieved by using a simple system based on the invention that incorporates a media holding tray that puts the print head in contact with the media and a supplemental surface. The combination of the surfaces which are in contact with the print head present a surface of uniform width (width that does not change as the disc is printed). This supplemental surface comprises a mask, that has a thickness and structural characteristics that are substantially the same as the media.
The invention described below consists of a thermal printer that utilizes a tray type of media holder with materials arranged in such a manner as to maintain a uniform print head pressure to media as the media moves relative to the print head.
The media to be printed is placed in the media tray which consists of a base layer of compressible material (mounted on either a platform or platen) and a second mask layer of material similar to the thickness and composition of the media. The mask layer has a cutout in which the media is positioned. This arrangement allows the printable surface of the media to be at the same level as the unmasked areas of the compressible surface.
The key feature of this arrangement is that as the print head passes over the media, the area of contact between the print head and the sum of the areas of the media and the surface of the media holder remains constant. This results in uniform (unchanging) print head pressure on the media during the entire printing process.
By careful selection of the materials of the media holder, the proper print head to media pressure can be maintained without the use of complex print head pressure control systems. In addition, proper print head pressure can be maintained when printing odd shaped, non-rectangular media, such as disc shaped objects, where the print head's area of contact with the media varies as the print head moves relative to the disc.
The base layer (compressible surface) and the mask layer (surface with cutout area in the shape of the media) may have one of more layers of material, so long as the surface of the mask layer has similar mechanical characteristics to the item being printed.
A typical composition of the base layer would consist of a material that compresses to the appropriate degree needed to maintain proper print head pressure distribution on the media. The preferred embodiment for the disc printing application would require a base layer material that has a compression value of 40-70 durometer which could include materials such as neoprene and other rubber-like substances.
A typical configuration of the mask layer would consist of a material that does not compress or has the same compression characteristics as the media. The preferred material for the mask layer of the disc printing application is a non-compressible material such as polycarbonate. CD-ROM and CD-R discs are typically made from molded polycarbonate.